"Seth Gottlieb has an excellent blog on content
management called . Recently, he had a post about The Onion's
adventures with Drupal. It's a story that illustrates the issues
companies face when highly customizing an open source content
management system. According to Gottlieb, when The Onion went with
Drupal 4.7, they had to "hack the core" to allow for the high
visitor volume they expected to their very funny and popular
website. What The Onion couldn't anticipate was that Drupal would
eventually solve this problem. The trouble was that by the time
they released updated versions, The Onion's hacked version was so
different from the original, it could no longer afford to upgrade
without sacrificing its substantial customizations.
"And that, in a nutshell, is one of the problems companies could
face when customizing open-source tools to this extent."